Patent GB-582503-A (GB)
Title: Improvements in or relating to pulse signal selecting and indicating systems
IPC: G01S13/70
Filed: 1943-10-15
Abstract: 582,503. Radiolocation; valve circuits. WILLIAMS, F. C., WHITE, E. L. C., and BLUMLEIN, D. [Representative of BLUMLEIN, A. D.]. Oct. 15, 1943, No. 17006. [Class 40 (v)] [Also in Group XXXV] Regularly recurrent electrical signals are rendered effective at a receiving point by locally generated signals or strobe pulses whose timing is varied to perform a searching operation over a period of time, the search being repeated continuously until coincidence in timing is obtained between the signal and strobe pulses, whereupon the searching operation is terminated and the subsequent timing of the strobe pulses is automatically controlled in accordance with the timing of the recurrent signals which are thereby rendered effective. The invention is particularly applicable to pulse modulated radiolocation systems in which it is desired to select one particular echo signal for display on a cathode ray tube indicator, only the signal coincident with a locally generated strobe pulse being fed to the indicator. The invention also enables undesired signals, such as ground echoes in a radiolocation system mounted in an aircraft, to be eliminated by comparing them with recurrent strobe pulses of predetermined duration and preventing them from producing an indication when their duration approaches that of the strobe pulses. A block diagram of an airborne radiolocation system embodying the invention is shown in Fig. 1. A transmitting dipole TA is coupled to a transmitter T which is pulse modulated by a modulator M, the grids of the transmitter oscillator valves being connected to a delay network which due to reflection causes each transmitted pulse to be rapidly terminated. The receiving aerial RA comprises four dipoles spaced apart to determine the azimuth and elevation of a reflecting object, the received signals being fed in sequence, through a rotary switch AS to the receiver comprising radio frequency amplifier RFA, frequency changer FC, I.F. amplifier and detector IFD and L.F. amplifier and output stage LF and V611. Signals are fed via lead R to the observer's indicator S whence they are displayed as deflections on the time base of a range cathode ray tube which is controlled by a synchronising pulse over lead A from the modulator. A second synchronising pulse B controls the remainder of the equipment and is fed through a pilot's control PS to a delay network DN 600 having two outputs feeding a suppression unit V601 and the strobe timing circuit V602. The suppression unit generates two negative pulses, one of which stops the receiver local oscillator via lead G during the transmitted pulse, while the other is fed via lead H to the R.F. and I.F. stages to relieve the automatic gain control circuit at close ranges. The strobe timing circuit V602 in conjunction with a drift circuit V615/6/7, controls the timing of the strobe pulse generator V604/5 which produces three pulses E.F. and D of selected duration and timing separation. Pulse E is fed to the observer's indicator unit S to provide a strobe marker on the range time base for facilitating normal searching and is also fed to the signal measuring rectifier V606 which is also fed with the receiver output over lead I. The signal measuring rectifier provides a rectified output representative of the signals received by the aerials RA, when the pulse E coincides with an echo, and this output is amplified at V614 and fed to the automatic gain control circuit V613 and also through the switch S601, which rotates synchronously with the aerial switch AS, to the deflecting plates of the parallel connected direction indicator cathode ray tubes in the pilot's and observer's indicators V1 and S. The position of the spot indication on these C.R. tubes is thus an indication of the relative amplitudes of the signals received by the different aerials RA and thus the direction in azimuth and elevation of the object producing the signals. The D.C. output from the A.G.C. circuit is mixed with the D.C. output from a noise limiter V610 fed from the receiver output stage V611, and is employed to reduce the gain of the I.F. amplifier in the event of high signal strength or noise level. Pulse D is fed together with the receiver output on lead K to the drift stop rectifier V607 which develops a voltage which is applied over lead L to the drift circuit V615/6/7 to interrupt the automatic searching and cause the strobe pulse D to become " locked " to a signal. The drift circuit also controls the range indicator circuit V619/20/1, via lead N, which causes range indicating " wings " to be produced on the spot indications on the direction indicator cathode ray tubes. At close ranges the circuit V619/20/1 is controlled by the output from the noise limiter V610 over lead O. The drift circuit also controls, via lead P, a blackout valve V612 which biasses off the direction indicator tubes during automatic searching. The pulse F which has a longer duration than pulse E is fed together with the receiver output on lead J to the ground echo rejection rectifier V608/9. Received pulses are compared with pulse F and if they are of longer duration than normal echo pulses from targets such as aircraft, a potential is built up which causes the drift circuit to recommence the searching cycle. DETAILED OPERATION OF CIRCUITS. Receiver output stage, Fig. 4. The receiver output valve V611 operates as a phase splitter, negative going signals from the anode being fed to the continuous wave interference and noise limiting valve V610. The cathode load comprises inductance L632 in series with resistance R658 and resistance-condenser combination C661, C662, C663, R653, R654, R655 so arranged that the load presents a substantially constant resistance at all frequencies, and a circuit comprising condenser C664 and resistance R657. The resistance-condenser combination has a time constant of approximately 3.4 microseconds so that echo signals which are approximately 2 microseconds long are passed while interfering signals of long pulse length are differentiated. The outputs from terminals t4 and t12 are fed to the range cathode ray tube via an amplifier (not shown) and to the signal measuring and drift stop rectifiers respectively. Condenser C664 and resistance R653 have a time constant of 20 microseconds and if the signal lasts as long as 5.5 microseconds indicating a ground echo, it is passed via terminal t8 to the ground echo rejection rectifier. Strobe pulse generating and timing circuit, Fig. 2. The strobe pulse generator comprises valves V604, V605 connected as a multivibrator, and the triggering action which occurs when the grid potential of V604 reaches an upper critical value of about 132 volts and the anode current of V605 is cut off, is employed to form the strobe pulses. The anode of V605 is connected to the delay network DN601 having a short-circuited end 6¢ sections from the input and the other end terminated in the characteristic impedance, 6 sections away from the input. The three pulses E, D and F are obtained respectively from the tapping points P2, P3 and P4 and are of duration and relative timing as indicated in Fig. 7. The timing of the strobe pulses which is determined by the time taken for the grid potential of V604 to move from a lower critical value (126 volts) at which valve V604 is cut off, downwards and then up again to the upper critical value (132 volts), is fixed by the timing valve V602. Valve V602 has applied to its control grid the positive synchronising pulse B obtained from the modulator via tapping P5 on the delay network DN600, Fig. 4. This 'pulse causes the anode voltage to fall approximately to the cathode potential which is always lower than the lower critical potential for the grid of valve V604. The rise of anode voltage at the end of the synchronising pulse, due to the resistancecondenser combination R609, R610, R612, C622, C623 and stray capacities, is initially rapid followed by a rise at a slower rate as shown in curves J1, J2, J3, Fig. 6, the time for the anode to rise to the critical value of 138 volts being set by the cathode potential of valve V602. Curves KI, K2, K3, Fig. 6 indicate the grid potentials of valve V605 corresponding to the curves J1, J2 and J3, the trailing edges of the grid voltage waveforms corresponding with the production of the strobe pulses. The voltage applied to the cathode of valve V602 which controls the timing of the strobe pulses is varied slowly from 125 to 25 volts, followed by a quick return to 125 volts under the control of valves V615, V616 and V617, forming the drift circuit. Valve V615 has a condenser C683 connected between its anode and grid and functions as a Miller sawtooth generator, the anode voltage falling at a steady rate between 125 and 25 volts in approximately 2 seconds. Due to the potentiometer comprising resistances R605, R606 and R711, R710, R737 and R728, Fig. 3 (terminals t2, Fig. 2 and t3, Fig. 3 being connected together), when the anode of V615 falls to 24 volts diode 4 of valve V609, Fig. 3, conducts and a negative potential is applied via terminals t9 and t10 to the grid of valve V617. Valve V617 is thus cut off and the anode voltage rise fed through resistances R693, R689 causes valve V616 to conduct, the resulting fall of anode voltage being applied to the suppressor grid of valve V615 which is thus cut off so that its anode potential rises rapidly as determined by resistances R605, R606 and condenser C624. This return stroke is terminated by the action of diode 5 of valve V618 which prevents the cathode voltage of valve V602 rising above 125 volts, and the searching cycle, with the anode voltage of valve V615 falling linearly, recommences. During the rise of anode voltage of valve V615 condensers C683, C685 and C667 are also charged and the voltage developed across resistance R689 by the charging of condenser C685 maintains a positive potential on the grid of valve V616 and hence the negative